feat(assertion): Add toBeDefined assertion method

ts/smartexpect.classes.assertion.ts

@@ -71,6 +71,22 @@ export class Assertion {
     }
   }
 
+  /**
+   * checks if the given object is defined
+   */
+  public toBeDefined() {
+    return this.runCheck(() => {
+      if (this.getObjectToTestReference() === undefined) {
+        throw new Error(
+          `${this.baseReference} with drill down ${this.propertyDrillDown} is not defined`
+        );
+      }
+    });
+  }
+
+  /**
+   * checks if the given object is not defined
+   */
   public toBeTypeofString() {
     return this.runCheck(() => {
       if (typeof this.getObjectToTestReference() !== 'string') {